In fabrication of semiconductor devices, increasing device density on a given die area provides significant advantages, including speed, power efficiency, and portability. To produce very fine resolution, very small device features and very small circuits are generated by a process of epitaxy, lithography, and etching. In typical processes, layers are deposited on the surface of a substrate and then masked and etched to a predetermined thickness. Lines of a material are then formed by depositing a layer of the material and then masking and etching away the material to form spaces where the layer is not desired. Depositing, masking and etching layers of substrate material allows the fabrication of very small semiconductor substrate features. However, because of uncertainties in the original thickness of a layer and uncertainties in etch rate, trace amounts of the material are occasionally left on the substrate. When the material is a conductor, for example, metal, the trace amounts of material often cause short circuits between the lines. Damascene techniques avoid this problem by depositing an insulating layer, forming trenches in the insulating layer, and then depositing the metal or other material within the trenches.
Damascene trenches are frequently formed by photolithographic patterning of the insulating layer, to achieve trench dimensions as small as possible. Photolithography produces very fine resolution on a substrate, but the resolution is limited by the particular wavelength used. But manufacturing constraints associated with the technologies capable of producing extremely high resolution, and in particular fine and closely spaced lines, hamper development of dies having greater device density. Damascene techniques therefore suffer from the same wavelength-dependent size limitations as other approaches to die manufacture. For deep-UV frequencies, each frequency carries a feature size limitation.
Presently, Deep-UV wavelengths are commonly used for fine-resolution lithography, and are considered state of the art. The E, G. H. and I lines of the deep-UV wavelengths are as short as 365 nm, which prevents the easy processing of a feature size finer than 248 nm (2480 .ANG.). Switching to a higher frequency would theoretically allow greater density, but would be prohibitively expensive due to a consequent need to develop new equipment, fabrication techniques, or resists appropriate to the shorter wavelengths, and the need to make presently available methods at the shorter wavelengths cost effective.
Some approaches to reducing feature size without switching to a higher frequency have been attempted. For example, phase-controlled interference patterns have been used with some success. These techniques place layers that are phase-shifting and transparent to the particular wavelength over (or under) some areas on a mask before exposing the mask, thus generating and positioning points of constructive and destructive interference on photoresistive materials. In addition to adding, large expense, these methods require broader areas of the device substrate to be exposed simultaneously, and the patterns desired must be highly repetitive for these methods to be appropriate. The added expense comes, in part, from the need to use materials and technologies matched to the resist and to the wavelength, and the material is difficult to align. Also, adjacent areas of constructive interference are often seen as a single large region, and are not resolved in some cases.